Note on Charge and Field Fluctuations

The field fluctuations which arise from the possibility of creating positron-electron pairs are computed. These fluctuations, which are inescapable in all field measurements, give a simple interpretation of Heisenberg's results on charge fluctuations, and of the divergences which appear in his calculations. The pair-induced fluctuations in the radiation field are in general small of order $\alpha$ compared to those which arise from the corpuscular character of the radiation.